Either / ROP threading macro. Clojury

either.clj

(defmacro either->
  [expr & clauses]
  (assert (zero? (rem (count clauses) 3)))
  (let [g (gensym)
        steps (map (fn [[test then else]] `(if (reduced? ~g)
                                             ~g
                                             (if (-> ~g ~test)
                                               (-> ~g ~then)
                                               (reduced (-> ~g ~else)))))
                   (partition 3 clauses))]
    `(let [~g ~expr
           ~@(interleave (repeat g) (butlast steps))]
       ~(if (empty? steps)
         g
         (last steps)))))


(either->
 x
 pred1 then1 else1
 pred2 then2 else2)

;;; expands to

(let [G__7782 x
      G__7782 (if (reduced? G__7782)
                G__7782
                (if (-> G__7782 pred1)
                  (-> G__7782 then1)
                  (reduced (-> G__7782 else1))))]
  (if (reduced? G__7782)
    G__7782
    (if (-> G__7782 pred2)
      (-> G__7782 then2)
      (reduced (-> G__7782 else2)))))